T he scientists of today have become accustomed to the extremely rapid pace of progress in the biomedical sciences spurred on by the discovery of recombinant DNA and the advent of automated DNA sequencing and PCR, with progress usually being measured in months or years at most. What is often forgotten, however, are the many prior advances that were needed to reach our present state of knowledge. Here I illustrate this by discussing the scientific discoveries made over the course of the past century and a half that ultimately led to the recent successful development of drugs, particularly imatinib mesylate, to treat chronic myelogenous leukemia.